1. Field of the Invention
The present invention relates to methods and apparatus for shaping and polishing (finishing) a surface; more particularly to methods and apparatus for shaping and polishing a surface by the impingement of a magnetically-modifiable and magnetically-directable jet; and most particularly to a magnetically-efficient nozzle for extruding a jet of magnetically-solidified magnetorheological fluid in an upwards direction.
2. Discussion of the Related Art
Fluid jets containing abrasive particles are known to be used for cutting or shaping materials such as glass, ceramics, plastics and metals. This technology is known generally as abrasive stream finishing, or abrasive suspension jet machining, or abrasive flow machining. Typically, such jets are impinged upon the substrate to be cut at a relatively high velocity, which may exceed 10 meters per second. When the jet strikes the impact zone, the abrasive particles in the fluid chip away particles of the substrate surface. The rate of material removal is a function of the kinetic energy of the jet, the sharpness, size, and hardness of the abrasive particles, the material of the substrate, the distance from the jet nozzle to the workpiece, and the angle of incidence of the jet.
In U.S. Pat. No. 5,971,835 issued Oct. 26, 1999 to Kordonski et al., the relevant disclosure of which is hereby incorporated by reference, a technology is disclosed by which a magnetorheological (MR) fluid may be formed into a substantially coherent abrasive jet. A continuous stream of an MR fluid is directed through a non-ferromagnetic tube disposed axially of the helical windings of an electric solenoid. The tube defines a nozzle. Preferably, the MR fluid is combined with a finely-divided abrasive material, for example, cerium oxide, diamond dust, or iron oxide, such that the abrasive is at least temporarily suspended therein. Flow of electricity through the solenoid creates an axial magnetic field within the windings which forms in the fluid a field-oriented structure of fibrils from the magnetic particles and thereby reversibly stiffens the flowing MR fluid into a virtually solid rod. The rod manifests a very high yield stress when sheared perpendicularly to the direction of flow and a relatively low shear stress when sheared in the direction of flow, as along the wall of the tube. Such anisotropic fibrillation allows the stiffened fluid to flow through the tube in the magnetic field. The MR rod ejected from the nozzle defines a highly-collimated, substantially solid jet of MR fluid. Upon leaving the nozzle, the exit of which is flush with the end of the windings, the MR fluid jet passes beyond the solenoid""s magnetic field, and the anisotropic fibrillation within the jet begins to decay. However, remanent high viscosity, and thus consequent stabilization of the MR jet, can persist for a sufficient time that the jet may travel up to several feet without significant spreading and loss of structure. This permits use of the abrasive jet to shape and/or polish the surface of a workpiece in a work zone at some distance from the nozzle.
At least three serious problems can arise in regard of the prior art apparatus.
First, the prior art apparatus is not suited to finishing deeply concave surfaces. Because of splashing, pooling, and gravitational effects, we have found that the optimal finishing attitude for the abrasive jet is directly upwards. However, some of the spent MR fluid rebounding from the surface of the workpiece falls back onto the solenoid and nozzle, clogging the exit and subsequently deforming the jet.
Second, the nozzle is a non-ferromagnetic axial tube in which the magnetorheological fluid is stiffened progressively as it flows through the nozzle, creating a progressively increasing viscous drag in the nozzle which must be overcome by the system""s pump. Thus, the pump and energy requirements for the prior art apparatus can become substantial.
Third, because the solenoid lacks a ferro-magnetic core, the axial magnetic field is relatively weak, requiring an undesirably large and expensive solenoid.
What is needed is a magnetorheological finishing apparatus which can direct a stiffened jet in any direction, and especially in an upwards direction, continuously without becoming fouled by reflected fluid; which has a small pump by virtue of developing minimal viscous drag in delivery of the stiffened jet; and which has a small, magnetically-efficient solenoid by virtue of having a ferromagnetic solenoid core.
It is a primary objective of the invention to provide means for delivering a jet of solidified magnetorheological fluid for abrasive finishing of deeply concave substrates.
It is a further object of the invention to provide a compact abrasive finishing apparatus having a small, inexpensive pumping system and a small solenoid.
Briefly described, in an apparatus for abrasive jet shaping and polishing of a surface using magnetorheological fluid, similar to the apparatus disclosed in U.S. Pat. No. 5,971,835, the non-ferromagnetic nozzle (shown as item 30 therein) within the solenoid is replaced by a nozzle formed of ferromagnetic material such that the fluid is magnetically shielded within the nozzle. The improved nozzle serves as a ferromagnetic core for the solenoid, thereby increasing the strength of the axial magnetic field approximately 100-fold and permitting a significant reduction in the required size of the solenoid. The exit orifice of the nozzle is recessed within the solenoid turnings, rather than being flush with the end of the solenoid as in the prior art apparatus, thus creating a free space within the solenoid having an intense axial magnetic field near the exit orifice of the nozzle. Stiffening of the magnetorheological fluid is prevented substantially throughout the length of the nozzle until the fluid begins to enter the magnetic field as it leaves the nozzle; thus, there is no buildup of viscous drag through the nozzle. Formation of fibrils and consequent stiffening of the jet occurs principally in free space within the windings of the solenoid. The exit end of the nozzle is configured so that the magnetic field at the end of the nozzle and in the free space immediately downstream of the exit is intensified and collimated. Further, the nozzle is provided with a radial array of longitudinal channels along its outer surface through which compressed air is injected to form a cylindrical air curtain which surrounds the jet as it emerges from the nozzle and solenoid. Returning MR fluid splashed from the workpiece is diverted by the air curtain and prevented from entering and fouling the solenoid exit and nozzle.